<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f1()">
<div>结果<span></span></div>

<script>
    function f1 (){
        let i = document.querySelector("#i1");
        let s = document.querySelector("span");
        /*isNaN()判断变量是否是NaN*/
        if(isNaN(i.value)){
            s.innerText = "输入错误";
            return;
        }
        s.innerText = i.value*i.value;
        //*当俩个字符串进行-*/运算时会自动转成数值，进行+运算时拼接
        //s.innerText = i.value*1+i+value*1;
        s.innerText = i.value+i.value;
        /* parseFloat()将字符串转成整数或小数*/
        s.innerText = parseFloat(i.value)+parseFloat(i.value);
    }
</script>
</body>

</html>